1. Field of the Invention
The present invention relates to a semiconductor integrated circuit that performs information processing by using a central processing unit (CPU), and, more particularly, to a semiconductor integrated circuit that is capable of transferring data at high speed to and from the outside.
2. Description of the Related Art
Conventionally, a CPU in a semiconductor integrated circuit that performs information processing is connected to a main memory via a bus, and is designed to transfer information to and from the outside (e.g., an external device or network) via the main memory. Because the access speed of the main memory is low (slow) in comparison with the processing speed of the CPU, the CPU is also connected to a cache memory that has a higher (faster) access speed than the main memory. The CPU accelerates information processing by temporarily storing a portion of the information stored in the main memory in the cache memory, and then delivering the information to and from the cache memory. An example of this kind of CPU is disclosed in Japanese Patent Kokai (Laid Open Publication) No. H8-180008.
In this semiconductor integrated circuit, when information is transferred to and from the external device via the main memory, the CPU is unable to transfer information at high speed because the main memory access speed is low. If the CPU has a cache memory, direct access to the cache memory from the external device may be considered. However, the cache memory requires the information stored in the cache memory to be the same as the information stored in the main memory. Therefore, it is not feasible to write and read information into and from the cache memory unless the same information is stored in the main memory.